The invention concerns a generalized method and apparatus for taking ranging data. The invention has special use as an Acoustic Seafloor Classification System, i.e. a system which can map contours of seabeds, riverbeds, and the like, and can identify the depth and constituent layers of the sediment, typically down to several meters.
An acoustic sediment classification system of this general kind was developed by the ELAC division of Honeywell Corporation for the Naval Ocean Research and Development Activity, now a part of the Naval Research Laboratory. The ELAC system has an acoustic transducer to send and receive acoustic pulses, a central signal processor and system controller which receives and processes digitized raw echo data, an analog recorder to record the raw data, and a central storage and data processing computer to store the data, and further process it in accordance with several look-up tables in computer memory. The controller computer causes the transducer to launch an acoustic pulse in the water toward the seabed (or riverbed, lake bed, etc.) and causes the system to listen for echoes of the pulse within preset time windows. Upon reaching the bottom, part of the pulse will reflect at the water-bottom interface, and part will transmit through, in proportion to the differences in acoustic impedance across the water-bottom interface. Similarly, at the interface between each layer of the bottom, similar reflection and transmission will occur. Thus responsive to the pulse from the transducer, a series of echoes will return, spaced in time and of varying amplitudes. The system listens for these echoes during a series of time windows in order to identify when each arrives, and further identifies the amplitude of the echoes in each time window. The data processing computer has a series of look up tables which use these data, to infer the acoustic reflectivity of the interfaces from which the echoes returned, the acoustic impedances of the layers themselves, and the depth of the interfaces. More on the ELAC system is given in the paper by D. N. Lambert, An Evaluation of the Honeywell ELAC Computerized Sediment Classification System, Naval Ocean Research and Development Activity Report 169 (August, 1988).
As the Report explains, the ELAC system has yielded important results, but also has significant drawbacks. In order to be sufficiently inexpensive to permit widespread use, as well as sufficiently portable (and user friendly) to take aboard ship, the computers used must be moderately sized, preferably PC's, and certainly no bigger than a modest workstation. Virtually all the active memory in such computers is in the form of Dynamic Random Access Memory, often called D-Ram. D-Ram's main virtue is its plentifulness and inexpensiveness. Its main handicap is that a computer using D-Ram must boost, i.e. "refresh," D-Ram memory frequently, or the memory fades. Although too rapid a process for a human viewer to notice in real time, a significant fraction of such a computer's time and effort is devoted to refreshing D-Ram, which correspondingly slows the computer's other operations. In the ELAC system, this impedes the flow of echo data into the system controller, and further impedes data flow between and through the system controller and the data processing computer.
Static Random Access Memory (S-RAM), i.e. memory which does not need periodic "refreshing," does exit, and if used in place of D-RAM in the ELAC or similar system, would speed processing considerably. Unfortunately, S-RAM is expensive, and would be prohibitively expensive to substitute for D-Ram in the computers of the ELAC system.
Additionally, the ELAC system has no provision for automatically associating with the echo data, the latitude and longitude at which the data were taken, further limiting the ELAC system's usefulness as a survey tool.